Spheroidal Graphite Cast Iron Excellent in Gas Defect Resistance

ABSTRACT

There is provided with spheroidal graphite cast iron having excellent gas defect resistance where gas defects such as pinholes attributable to the free N are small in number and having mechanical characteristics and machinability equal to or greater than the conventional ones. The spheroidal graphite cast iron consists of, in mass ratio, C: 3.3 to 4%; Si: 2 to 3%; P: not more than 0.05%; S: not more than 0.02%; Mn: not more than 0.8%; Cu: not more than 0.8% (0 is not included); Mg: 0.02 to 0.06%; Ti: 0.01 to 0.04%; V: 0.001 to 0.01%; Nb: 0.001 to 0.01%; and N: 0.004 to 0.008%, with the remnant substantially consisting of Fe and an inevitable impurity.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national phase under 35 U. S. C. § 371 of PCTInternational Application No. PCT/JP2016/055973 which has anInternational filing date of Feb. 29, 2016 and designated the UnitedStates of America.

FIELD

The present invention relates to a spheroidal graphite cast ironexcellent in gas defect resistance.

BACKGROUND

Spheroidal graphite cast iron which has excellent mechanicalcharacteristics and good castability is widely used for variousautomobile parts and machine parts. In the manufacture of the spheroidalgraphite cast iron, as main raw materials, pig iron, steel scrap, returnscrap (casting return material) and the like are used. Of theabove-mentioned raw materials, pig iron for castings was conventionallythe main material as the raw material of the spheroidal graphite castiron; however, in recent years, from the viewpoint of economical use ofresources, it has been common to mainly use steel scrap instead of pigiron the price of which has risen. Steel scrap has been more and moreheavily used as a raw material of cast iron in general includingspheroidal graphite cast iron since it is caused in large amount withthe growth of the automobile industry and is supplied inexpensively.

Here, the steel scrap caused in the automobile industry is formed ofmachining scrap for automobile bodies and the like, and in recent years,as the steel material thereof, the ratio of high-tensile steel plateshas been increasing. The reason therefor is that for automobile bodiesand the like, while weight reduction is required from the viewpoint ofenvironmental preservation by improvement of fuel efficiency, securementof strength and rigidity is also required from the viewpoint ofsecurement of passengers' safety at the time of a collision. For thisreason, it is required that automobile bodies and the like attain bothweight reduction and strength enhancement/rigidity enhancement, and torespond thereto, high-tensile steel plates containing high levels of Mn,Cr, Mo and the like tend to be used heavily. When steel scrap inevitablycontaining a large amount of Mn, Cr, Mo and the like is used as the rawmaterial, a problem arises in that ductility (elongation characteristic)is deteriorated since the crystallization of graphite is inhibited or acarbide is generated when it is made into spheroidal graphite cast iron.To solve the problem attributable to Mn, Cr, Mo and the like inevitablycontained in the raw material, various proposals have been made toremove these elements from the melt in the melting process of spheroidalgraphite cast iron.

Some high-tensile steel plates are enhanced in strength by containing N(nitrogen) in addition to Mn, Cr and Mo and undergoing precipitationhardening with a carbonitride or by undergoing nitriding. Some of thesehigh-tensile steel plates contain as much as several hundreds of ppm ofN. When spheroidal graphite cast iron is manufactured by using, as theraw material, steel scrap inevitably containing a large amount of N likethis, there is a possibility that free N contained in the melt intowhich this raw material is melted increases. In the followingdescription, “free N” indicates a nitrogen atom in a free state notconstituting an atom constituting a solid phase or a solid solution, and“N” indicates nitrogen as an element.

Moreover, generally, the melt (raw melt) formed in the melting processof spheroidal graphite cast iron is deoxidized at the stage of themelting process since it contains Si and Mn, and thereafter, is furtherthoroughly deoxidized by the Mg added in the spheroidizing processingperformed after the raw melt is shifted to a ladle and the Si added inthe inoculation processing. The thus deoxidized melt for spheroidalgraphite cast iron has the property of readily absorbing or occludingthe free N in the atmosphere when the melt is exposed to the atmospherein the process of tapping from a melting furnace into a ladle or in theprocess of pouring from the ladle into a mold.

In the melt for spheroidal graphite cast iron having the property ofreadily absorbing or occluding the free N derived from the atmosphere asdescribed above, when the content of steel scrap with a high N contentinevitably containing a large amount of N as the raw material furtherincreases, the free N in the melt increases because of this free Nderived from steel scrap, so that the tendency increases that a gasdefect such as a pinhole formed of nitrogen gas (N₂ gas) attributable tothe free N is caused in the cast products. Specifically, when a meltexcessively containing free N solidifies, there are cases where the freeN that cannot be solid-solved is released as nitrogen gas into the solidphase to cause a gas defect such as a pin hole formed of nitrogen gas inthe cast products. If a gas defect is caused in the spheroidal graphitecast iron, not only a poor appearance but also a problem can arise inthat deterioration of mechanical characteristics such as strength andelongation is caused by the minute hole defect.

As a technology of reducing the content of impurities containing Nimmixed into cast iron such as spheroidal graphite cast iron, JapanesePatent Application Laid-Open No. 2004-169167 has a description of aspheroidal graphite cast iron containing, in mass ratio, 2.0 to 4.0%carbon, 0.01 to 0.1% spheroidization promoting element formed of one ormore than one of magnesium, calcium and a rare-earth element, 1.0 to3.0% silicon and not more than 0.02% sulfur with the remnant consistingof iron and inevitable impurities and in which the amount of elementsother than cobalt, copper and nickel in the inevitable impurities ismade as small as possible so that the absorption energy value in aCharpy impact test at −60 degrees C. or −80 degrees C. is not less than14 J/cm² on a V notch test piece by doing the following: An alloyhaving, in mass ratio, 2.0 to 4.0% carbon, 1.0 to 3.0% silicon and notmore than 0.02% sulfur with the remnant consisting of iron and a verysmall amount of inevitable impurities is introduced into a cold cruciblemelting furnace and melted and then, a spheroidizing agent containing aspheroidization promoting element formed of one or more than one ofmagnesium, calcium and a rare-earth element is added thereto so that theratio of the spheroidization promoting element is 0.01 to 0.1% in thefinal composition, cooling is performed without inoculation processingto promote graphitization being performed and a manufacturing method toproduce an alloy with iron is applied.

According to Japanese Patent Application Laid-Open No. 2004-169167, inmelting the raw material, after the melting furnace chamber isevacuated, argon gas is introduced to make the inside of the meltingfurnace chamber an argon atmosphere and by using a water-cooled highlypure copper crucible and using a cold crucible melting furnace which isan apparatus that simulatively brings the copper crucible and the moltenmetal into a non-contact state and melts them (levitation melting), theimmixture of impurities from the crucible or the gas phase (environment)into the molten metal as in the conventional melting process isprevented and a highly pure material can be produced. Moreover, in thespheroidal graphite cast iron of Japanese Patent Application Laid-OpenNo. 2004-169167, by making not more than 0.003% in mass ratio the ratioof each of the elements other than cobalt, copper and nickel in theinevitable impurities, the inclusion can be further reduced and aspheroidal graphite cast iron where the internal brittle part is reducedcan be provided.

SUMMARY

In the method of manufacturing spheroidal graphite cast iron disclosedin Japanese Patent Application Laid-Open No. 2004-169167, since the rawmaterial is melted in the argon atmosphere and solidified as it is toform spheroidal graphite cast iron, the free N contained in the melt isreduced and consequently, there is a possibility that the gas defectattributable to the free N caused in spheroidal graphite cast iron issuppressed. However, highly pure electrolytic iron of, for example,approximately 4N (99.99%, mass ratio) level, silicon for semiconductors,highly purified graphite and the like are used as the raw materials, andthe starting materials themselves are highly pure raw materials.

Furthermore, the manufacturing method thereof is provided by making theinside of the melting furnace chamber the argon atmosphere and using acold crucible melting furnace which is a special apparatus formed of awater-cooled highly pure copper crucible. The cold crucible meltingfurnace is generally used for the manufacture of a highly pure materialsuch as the manufacture of a highly pure alloy ingot. When a highly pureraw material and a special apparatus are used as described above,although the content of inevitable impurities can be significantlyreduced, for the manufacture of spheroidal graphite cast iron applied toautomobile parts and machine parts, the obtained cast products areextremely expensive, which is a problem from the viewpoint of economicrationality.

Moreover, according to the manufacturing method of Japanese PatentApplication Laid-Open No. 2004-169167, after the raw material is meltedin the copper crucible and the spheroidizing agent is added, cooling andsolidification of the melt are performed in the copper crucible.According to this manufacturing method, the obtained spheroidal graphitecast iron is a massive cast product of a shape copying the shape of thespace in the crucible. With this, advantages of casting which is a workmethod having a high shape freedom degree in nature cannot be enjoyed.

That is, to obtain cast products having free shapes such as automobileparts and machine parts, it is necessary to perform cooling andsolidification after pouring into a mold having a cavity defining theproduct shape directly from the melting furnace or through a ladle. Tomanufacture cast products at an industrially realistic and rationalcost, it is important that the process of tapping from the meltingfurnace into the ladle and the process of pouring from the ladle intothe mold can be performed in the atmosphere.

Although it is considered to dispose a ladle and a mold inside thechamber and perform tapping and pouring after making the inside of thechamber an argon gas atmosphere or vacuous in order to suppresspenetration (absorption, occlusion) of free N into the melt, theequipment and apparatus are special and large-scale ones and theobtained cast products are further expensive, which is neithereconomical nor realistic. In addition, there is no effective andpractical method to remove the free N from the melt of spheroidalgraphite cast iron, and even if dilution with a material with a low Ncontent is attempted, the low N raw material such as highly pure pigiron and a base metal is expensive and uneconomical.

In recent years, the situation has been such that a large amount of N isunavoidably contained as an inevitable content in the raw material ofspheroidal graphite cast iron as described above. Moreover, in additionto steel scrap and the atmosphere which are typical examples of originsof the free N as described above, there exists plenty of origins of thefree N that can be dissolved in the melt such as other materialsconstituting the raw material and a furnace material of the meltingfurnace that melts the raw material. For this reason, it is industriallyvery effective if a spheroidal graphite cast iron can be obtained inwhich, for example, without the free N derived from steel scrap beingremoved or dilution with expensive pig iron or the like being performed,no gas defect such as a pin hole attributable to this free N is causedwhile the free N is contained in the melt.

The present invention is made in view of the above-describedconventional problems, and an object thereof is to provide spheroidalgraphite cast iron having excellent gas defect resistance where gasdefects such as pinholes attributable to the free N are small in numberand having mechanical characteristics and machinability equal to orgreater than the conventional ones.

That is, a spheroidal graphite cast iron excellent in gas defectresistance of the present invention consists of, in mass ratio:

C: 3.3 to 4%;

Si: 2 to 3%;

P: not more than 0.05%;

S: not more than 0.02%;

Mn: not more than 0.8%;

Cu: not more than 0.8% (0 is not included);

Mg: 0.02 to 0.06%;

Ti: 0.01 to 0.04%;

V: 0.001 to 0.01%;

Nb: 0.001 to 0.01%; and

N: 0.004 to 0.008%,

with the remnant substantially consisting of Fe and an inevitableimpurity.

Preferably, the spheroidal graphite cast iron of the present inventioncontains, in mass ratio, 0.015 to 0.045% Ti, V and Nb in total andfurther, contains Ti, V, Nb and N so as to satisfy the followingexpression (1):

0.8≤(0.29Ti+0.27V+0.15Nb)/N≤2.0  (1).

Here, the element symbols in the expression (1) represent the contents[mass ratio (%)] of the elements in the spheroidal graphite cast iron.

Preferably, the spheroidal graphite cast iron of the present inventioncontains, in mass ratio, not less than 0.005% P and not less than 0.005%S or/further contains, in mass ratio, not less than 0.2% Mn and not lessthan 0.1% Cu.

Preferably, the spheroidal graphite cast iron is not less than 600 MPain tensile strength and not less than 12% in elongation.

The spheroidal graphite cast iron of the present invention has excellentgas defect resistance with few gas defects such as pinholes attributableby the free N.

The above and further objects and features will more fully be apparentfrom the following detailed description with accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic plan view of a flat test piece for measuring agas defect.

FIG. 1B is a schematic side view of the flat test piece for measuring agas defect.

DETAILED DESCRIPTION

The composition of the spheroidal graphite cast iron of the presentinvention will be described below in detail. The contents of theelements constituting the alloy are expressed by mass ratios (%) unlessotherwise specified. Moreover, while in the embodiment described below,reduction in the gas defect attributable to the free N derived fromsteel scrap is described as an embodiment of the present invention, thepresent invention is not limited to the embodiment described below.

(1) C (Carbon): 3.3 to 4%

C is an element that contributes to the flowability of the melt and thecrystallization of graphite. When the C content is less than 3.3%, theflowability at the time of casting decreases and the number of graphitegrains decreases so that chill (Fe₃C: cementite) is apt to be formed,which decreases the elongation of the spheroidal graphite cast iron. Onthe other hand, when the C content exceeds 4%, shrinkage cavity is aptto appear and abnormal graphite is apt to be formed, so that strengthdecreases. For this reason, the C content is made 3.3 to 4%. The lowerlimit of the C content is preferably 3.4%, and is more preferably 3.6%.Moreover, the upper limit of the C content is preferably 3.9%, and ismore preferably 3.8%.

(2) Si (Silicon): 2 to 3%

Si is necessary for promoting the crystallization of graphite andenhancing the flowability of the melt. When the Si content is less than2%, chill is apt to be formed, so that the machinability and elongationof the spheroidal graphite cast iron decrease. However, when the Sicontent exceeds 3%, the matrix of the spheroidal graphite cast ironbecomes brittle, so that ductility (impact value) and elongationextremely decrease and strength and machinability deteriorate. For thisreason, the Si content is made 2 to 3%. The lower limit of the Sicontent is preferably 2.1%, and is more preferably 2.2%. Moreover, theupper limit of the Si content is preferably 2.9%, and is more preferably2.8%.

(3) P (Phosphorus): Not More than 0.05%

P is an element inevitably immixed from the raw material. P inhibits thespheroidization of graphite, and is solid-solved in the matrix toembrittle the structure. For this reason, the P content is made not morethan 0.05%. On the other hand, although no lower limit is set, since itis not economical to reduce it, for example, to not more than thedetection limit, it is desirable that the lower limit thereof beapproximately 0.005%. The upper limit of the P content is preferably0.03%.

(4) S (Sulfur): Not More than 0.02%

S is an element inevitably immixed from the raw material. S is agraphite spheroidization inhibiting element, and the content thereof ismade not more than 0.02%. On the other hand, although no lower limit isset, since it is not economical to reduce it, for example, to not morethan the detection limit, it is desirable that the lower limit thereofbe approximately 0.005%. The upper limit of the S content is preferably0.01%.

(5) Mn (Manganese): Not More than 0.8%

Mn, which is an element inevitably immixed from the raw material,precipitates a pearlite phase as a pearlite phase stabilizing element.When a spheroidal graphite cast iron the strength of which is improvedby stably precipitating a pearlite phase in the matrix structure isobtained, it is preferable that the Mn content be not less than 0.2%. Onthe other hand, when the content thereof exceeds 0.8%, the formation ofchill is conspicuous, so that the ductility, elongation andmachinability of the spheroidal graphite cast iron are deteriorated. Forthis reason, the Mn content is made not more than 0.8%. The upper limitof the Mn content is preferably 0.5%.

(6) Cu (Copper): Not More than 0.8% (0 is not Included)

Cu, which is a pearlite phase stabilizing element, is an element that iseffective when a spheroidal graphite cast iron containing a pearlitephase in the matrix structure and having its strength improved isobtained. For this reason, it may be contained in an appropriate amountwithout 0% included according to a desired strength. To stably generatea pearlite phase, it is preferable that the Cu content be not less than0.1%. However, when the Cu content exceeds 0.8%, the spheroidal graphitecast iron becomes too hard, and graphite spheroidization is inhibited,so that the elongation and ductility of the spheroidal graphite castiron decrease. For this reason, the Cu content is set to not more than0.8%. The upper limit of the Cu content is preferably 0.6%.

(7) Mg (Magnesium): 0.02 to 0.06%

Mg is an element necessary for graphite spheroidization that isimportant in improving mechanical characteristics such as the strengthand elongation of the spheroidal graphite cast iron, and when thecontent thereof is less than 0.02%, the effect of graphitespheroidization is insufficient. On the other hand, when the Mg contentexceeds 0.06%, chill and shrinkage cavity are apt to be formed, so thatthe machinability and ductility of the spheroidal graphite cast irondecrease. For this reason, the Mg content is made 0.02 to 0.06%. Thelower limit of the Mg content is preferably 0.025%, and is morepreferably 0.03%. Moreover, the upper limit of the Mg content ispreferably 0.05%, and is more preferably 0.04%.

(8) Ti (Titanium): 0.01 to 0.04%, V (Vanadium): 0.001 to 0.01%, Nb(Niobium): 0.001 to 0.01%

Ti, V and Nb are essential elements that are most important asconstituents constituting the spheroidal graphite cast iron of thepresent invention. From the viewpoint of being the structural elementsof the spheroidal graphite cast iron obtained by the melt, the free Ncontained in the melt of the spheroidal graphite cast iron is consideredto be finally released as nitrogen gas without being fixed (1) in thematrix phase, (2) in a nitride or a carbonitride or (3) to thestructural elements of the above (1) and (2) and exist as N mainly inthese three structural elements in the gas defect formed of the nitrogengas. The upper limit of the amount of N that can be contained in thematrix phase of (1) is the solid solubility limit of the austenite phasegenerated in the neighborhood of 1000 degrees C. which is theaustenitizing temperature.

Ti, V and Nb are powerful carbonitride forming elements (hereinafter,the three elements of Ti, V and Nb will sometimes be referred to ascarbonitride forming elements), and by these elements being contained innot less than a predetermined amount, a nitride or a carbonitride(hereinafter, these will sometimes be collectively called carbonitride)is formed by a chemical combination with the free N in the melt. Thecrystallization temperature of the carbonitride formed by theabove-mentioned carbonitride forming elements is higher than thecrystallization start temperature of the austenite phase (matrix phase),and in the process of the melt being cooled to solidify, thecarbonitride is formed earlier than the matrix phase. As a result, thefree N excessively dissolved in the melt beyond the solid solubilitylimit of N of the austenite phase is fixed as a carbonitride. Thereby,even in a spheroidal graphite cast iron formed by using a raw materialcontaining N more than desired, the free N exceeding the solidsolubility limit of the austenite phase is inhibited from being releasedas nitrogen gas from inside the melt at the time of solidification, sothat the occurrence of the gas defect is prevented.

In addition to the above-described effects of preventing the occurrenceof the gas defect, an effect can also be produced in that variation inthe mechanical characteristics of the obtained spheroidal graphite castiron can be reduced by suppressing variation in the amount of Nsolid-solved in the austenite phase by the fixing of the free N in themelt by the carbonitride forming elements. That is, N is a powerfulpearlite phase stabilizing element together with Mn and Cu, and promotesthe precipitation of the pearlite phase from the austenite phase. When asteel scrap with little N is used as has conventionally been done, theamount of N contained in the raw material among the melting lots(charges) is comparatively uniform, and variation in the free N in themelt dissolved in the raw material is small. For this reason, to obtaina desired pearlite phase, the precipitation amount of the pearlite phasecan be controlled by adjusting the additive amounts of elements such asMn and Cu the contents of which can be comparatively easily controlled.

However, when a steel scrap with a large amount of N is used as the rawmaterial in addition to the steel scrap with little N, the amount of Ncontained in the raw material among the melting lots varies according tothe composition of the steel scrap contained in the raw material, andthe variation in the amount of free N in the melt is also large. Forthis reason, the amount of N solid-solved in the austenite phase alsovaries and the precipitation amount of the pearlite phase is unstable,which causes variation in the mechanical characteristics (strength andelongation) of the spheroidal graphite cast iron among the melting lots.On the other hand, in the present invention, pearlitization by N issuppressed by reducing the amount of N solid-solved in the austenitephase by the fixing of the free N by the carbonitride forming elementsas described above. Thereby, the precipitation amount of the pearlitephase can be stably adjusted by controlling the contents of Mn and Cu,so that variation in the mechanical characteristics of the spheroidalgraphite cast iron can be reduced.

To obtain the effect of fixing the free N by the carbonitride formed bythe above-mentioned carbonitride forming elements, it is necessary thatthe contents of Ti, V and Nb be not less than 0.01%, not less than0.001% and not less than 0.001%, respectively. On the other hand, whenthe contents of Ti, V and Nb exceed 0.04%, 0.01% and 0.01%,respectively, an extremely hard carbide or nitride is formed, so thatthe machinability and mechanical characteristics (strength andelongation) of the spheroidal graphite cast iron decrease. For thisreason, the Ti content is made 0.01 to 0.04%, the V content is made0.001 to 0.01%, and the Nb content is made 0.001 to 0.01%.

The lower limit of the Ti content is preferably 0.012%, and is morepreferably 0.013%. Moreover, the upper limit of the Ti content ispreferably 0.035%, and is more preferably 0.025%.

The lower limit of the V content is preferably 0.002%. Moreover, theupper limit of the V content is preferably 0.004%, and is morepreferably 0.003%.

The lower limit of the Nb content is preferably 0.002%, and is morepreferably 0.004%. Moreover, the upper limit of the Nb content ispreferably 0.006%, and is more preferably 0.005%.

By the carbonitride forming elements being contained in a predeterminedamount as described above and fixing the free N as a carbonitride, aspheroidal graphite cast iron can be obtained that has an excellent gasdefect resistance with few gas defects such as pinholes attributable bythe free N and further, has mechanical characteristics (strength andelongation) and machinability equal to or greater than the conventionalones by the variation in its mechanical characteristics being reducedand the excessive carbonitride formation being suppressed.

Further, one significant feature of the present invention is that Ti, Vand Nb are not contained singly but these three elements are containedin combination and the contents thereof are controlled to appropriateamounts. By Ti, V and Nb being all contained in the above-mentionednumerical ranges as described above, the total amount of thesecarbonitride forming elements can be reduced more than when these arecontained singly or only two kinds are contained. Specifically, informing the same amount of carbonitride, by the above-mentioned threekinds of carbonitride forming elements being contained in combination,the total amount of contained carbonitride forming elements can besuppressed compared with when they are contained singly or only twokinds are contained. Thereby, the amount of carbonitride that affectsthe mechanical characteristics and machinability is made in anappropriate range while the above-described effect of fixing the free Nby the carbonitride formed by the carbonitride forming elements issufficiently produced, and consequently, a spheroidal graphite cast ironcan be obtained where both gas defect resistance, and mechanicalcharacteristics and machinability are obtained.

(9) Ti, V and Nb: Preferably 0.015 to 0.045% in Total

The total amount of contents of Ti, V and Nb in combination can be in arange of 0.012 to 0.06% from the total amount of upper limits and lowerlimits of the respective elements. To suppress the occurrence of the gasdefect by fixing the free N as a carbonitride and make more conspicuousthe effects of reducing the mechanical characteristic variation in arange of the N content described later, it is preferable that the totalcontent of Ti, V and Nb be not less than 0.015%. On the other hand, whenthe total amount of Ti, V and Nb exceeds 0.045%, a tendency to form ahard carbide or nitride increases, so that the deterioration of themachinability and mechanical characteristics (strength and elongation)of the spheroidal graphite cast iron is conspicuous. Therefore, thetotal content of Ti, V and Nb is made 0.015 to 0.045%. The lower limitof the total content of Ti, V and Nb is preferably 0.02%. Moreover, theupper limit of the total content of Ti, V and Nb is preferably 0.03%.

(10) N (Nitrogen): 0.004 to 0.008%

N is an element immixed mainly from steel scrap such as a high-tensilesteel plate. The melt of the spheroidal graphite cast iron obtainedthrough a melting process with such steel scrap as the raw materialcontains approximately 0.008 to 0.015% free N. Even in the spheroidalgraphite cast iron obtained by using a melt with plenty of free N likethis, by the carbonitride forming elements being contained in apredetermined amount as described above, the free N in the melt is fixedto the carbonitride formed by the carbonitride forming elements. As aresult, the content of N contained in the spheroidal graphite cast iron,together with the N fixed (solid-solved) to the matrix phase, is notless than 0.004%. On the other hand, when the N content is less than0.004% in spite of a spheroidal graphite cast iron obtained by using amelt with plenty of free N, there is a possibility that at the time ofcasting, excessive N that cannot be solid-solved is released as nitrogengas into the solid phase when the melt solidifies and this causes a gasdefect such as a pin hole in the spheroidal graphite cast iron. For thisreason, the N content is made not less than 0.004%. On the other hand,when the N content exceeds 0.008%, the carbonitride fixing N alsoincreases, so that there is a possibility that the machinability andmechanical characteristics (strength and elongation) of the obtainedspheroidal graphite cast iron decrease. For this reason, the N contentis made not more than 0.008%. Therefore, the N content is made 0.004 to0.008%. The upper limit of the N content is preferably 0.007%, and ismore preferably 0.006%.

(11)

0.8≤(0.29Ti+0.27V+0.15Nb)/N≤2.0  Expression (1):

In the spheroidal graphite cast iron of the present invention, tofurther improve the gas defect resistance and further improve themechanical characteristics (strength and elongation) and machinability,it is preferable to satisfy the expression (1) with the above-describedcomposition range requirements being satisfied. The element symbols inthe expression (1) represent the contents [mass ratio (%)] of theelements in the spheroidal graphite cast iron. Since Ti, V and Nb whichare carbonitride forming elements are united with N in one-to-onecorrespondence in the number of atoms, if the ratio between the totalamount of substance (T) of carbonitride forming elements shown in thefollowing expressions (2) and (3) and the amount of substance (N) of N,that is, the amount of substance ratio (molar ratio) T/N is within apredetermined range, the balance between the carbonitride formingelements and N is appropriate, so that the total content of Ti, V and Nbis one that is necessary and sufficient for the N content. By the amountof substance ratio (molar ratio) T/N being within a predetermined range,in the spheroidal graphite cast iron obtained by using a melt withplenty of free N, the formation of excessive carbonitride is alsosuppressed while N exceeding the solid solubility limit of the austenitephase is fixed as a carbonitride by Ti, V and Nb, so that the gas defectresistance, the mechanical characteristics (strength and elongation) andthe machinability are further improved.

T=(Ti/48)+(V/51)+(Nb/93)  (2)

N=N/14  (3)

The amount of substance ratio T/N between the total amount of substance(T) of the carbonitride forming elements and the amount of substance (N)of N, which is straightened up in consideration of the atomic weight, is(0.29Ti+0.27V+0.15Nb)/N in the expression (1). The coefficients by whichthe carbonitride forming elements are multiplied are coefficientsobtained from the ratio between the atomic weightf N and the elements;0.29 represents the ratio between the atomic weights of N and Ti(14/48), 0.27 represents the ratio between the atomic weights of N and V(14/51), and 0.15 represents the ratio between the atomic weights of Nand Nb (14/93).

When the value of the expression (1) is not less than 0.8, since anappropriate amount of carbonitride forming elements for N is contained,even in a spheroidal graphite cast iron obtained by using a melt withplenty of free N, N exceeding the solid solubility limit of theaustenite phase is fixed neither too much nor too little by thecarbonitride forming elements, so that sufficient gas defect resistanceis obtained. On the other hand, when the value of the expression (1) isnot more than 2.0, the formation of a carbonitride is suppressed to theminimum, so that the mechanical characteristics (strength andelongation) and machinability improve. Therefore, in the spheroidalgraphite cast iron of the present invention, it is preferable that thevalue of (0.29Ti+0.27V+0.15Nb)/N be in a range of 0.8 to 2.0.Theoretically, it is assumed that when the amount of substance ratiobetween Ti, V and Nb, and N in the spheroidal graphite cast iron is 1,that is, the value of the expression (1) is 1, N is formed as acarbonitride neither too much nor too little and there is nosolid-solving of N into the austenite phase; however, when inconsideration of the yield of formation of the carbonitride, thepromotion of precipitation of the pearlite phase by appropriatesolid-solving of N into the austenite phase, and the point that two freeNs (atoms) are necessary for molecularization as nitrogen gas, inactuality, a range of 0.8 to 2.0 is suitable. The value of the left sideof the expression (1) is more preferably 1.0, and is most preferably1.2. Moreover, the value of the right side of the expression (1) is morepreferably 1.7, and is most preferably 1.5.

(12) Mechanical Characteristics

Regarding the mechanical characteristics of the spheroidal graphite castiron of the present invention, it is preferable that the tensilestrength be not less than 600 MPa and the elongation be not less than12%. The spheroidal graphite cast iron having mechanical characteristicswhere the tensile strength is not less than 600 MPa and the elongationis not less than 12% is suitable for use for constructional members andthe like similarly to the conventional spheroidal graphite cast ironsince it has mechanical characteristics equal to or greater than theconventional ones. The tensile strength is more preferably not less than610 MPa, and is most preferably not less than 620 Pa. Moreover, theelongation is more preferably not less than 13%, and is most preferablynot less than 14%. To make the tensile strength not less than 600 MPaand make the elongation not less than 12%, it is preferable to adjustthe additive amounts of elements such as Mn and Cu the contents of whichare comparatively easily controlled, and specifically, it is preferablethat the Mn content be 0.2 to 0.5% and the Cu content be 0.2 to 0.6%.

Examples

While the present invention will be described in more detail by thefollowing examples, the present invention is not limited to theseexamples. In the following, the contents of the elements constitutingthe spheroidal graphite cast iron are also expressed in mass ratio (%)unless otherwise specified. Moreover, the Examples described below areexamples within the range of the present invention, the Comparativeexamples are examples outside the range of the present invention, andthe Reference example is an example representative of the reference ofthe conventional technology.

A melt was formed by melting pig iron, steel scrap and return scrap ofspheroidal graphite cast iron as the raw material in a high frequencymelting furnace with a capacity of 100 kg and adding a recarburizingagent and Fe—Ti, Fe—V, Fe—Nb and Fe—Si alloys for ingredient adjustment.The steel scrap was a high-tensile steel plate with an N content of0.05%, and of the compounded amount ratio 100% of the raw material whichwas the total of the pig iron, the steel scrap and the return scrap, thecompounded amount ratio of the steel scrap was 40% in first to sixteenthExamples 1 to 16 and Comparative examples 1 to 11 described later, andwas 0% in the Reference example where they were not compounded. Thismelt was tapped at approximately 1500 degrees C. into a ladle where anFe—Si—Mg alloy and a cover member made of steel scrap covering this wereplaced as a graphite spheroidizing agent, and spheroidizing processingby a sandwiching method was performed. The amount of N (free N)contained in the melt having undergone the spheroidizing processing wasin a range of 0.005 to 0.009% in all the Examples and Comparativeexamples described below, and was 0.003% in the Reference example. Themelt having undergone the spheroidizing processing was poured into asand mold at approximately 1400 degrees C., and was cast into aplurality of one-inch Y blocks, a mold for a flat test piece for a gasdefect area ratio evaluation described later and a mold for acylindrical test piece for a machinability evaluation. At the time ofpouring, Fe—Si alloy powder was added to the flow of the melt to performinoculation.

In the above-described manner, spheroidal graphite cast irons having thecompositions shown in Table 1 were obtained. The Examples 1 to 16 (Ex.1to Ex.16) are spheroidal graphite cast irons within the compositionrange defined by the present invention, and the Comparative examples 1to 11 (Com.1 to Com.11) and the Reference example (Ref.) are spheroidalgraphite cast irons outside the composition range defined by the presentinvention. The Comparative example 1 and the Comparative examples 7 to11 are spheroidal graphite cast irons where the content of one or morethan one element of Ti, V, Nb and N is too large, and the Comparativeexamples 2 to 6 and the Reference example are spheroidal graphite castirons where the content of one or more than one element of Ti, V, Nb andN is too small. The compositions of the obtained spheroidal graphitecast irons were checked by a glow-discharge mass spectrometer(manufactured by VG, the trade name: VG9000, hereinafter, referred tomerely as GDMS). The GDMS cannot measure, of the N existing in theabove-described structural elements, the N contained in the gas defectof (3). Therefore, the amount (mass ratio) of N shown in Table 1 is theamount of N solid-solved in the matrix phase of (1) and N fixed to thecarbonitride of (2).

TABLE 1 Composition (mass ratio (%))⁽¹⁾ No. C Si P S Mn Cu Mg Ti V Nb NEx. 1 3.55 2.45 0.018 0.010 0.35 0.41 0.035 0.015 0.002 0.005 0.004 Ex.2 3.65 2.35 0.021 0.009 0.42 0.37 0.034 0.035 0.006 0.002 0.007 Ex. 33.66 2.38 0.025 0.008 0.44 0.38 0.036 0.040 0.009 0.010 0.008 Ex. 4 3.562.41 0.028 0.009 0.41 0.39 0.038 0.037 0.004 0.002 0.006 Ex. 5 3.68 2.400.022 0.007 0.38 0.41 0.036 0.037 0.005 0.007 0.007 Ex. 6 3.57 2.390.019 0.008 0.45 0.40 0.039 0.025 0.004 0.006 0.006 Ex. 7 3.60 2.370.017 0.009 0.46 0.39 0.034 0.020 0.003 0.005 0.006 Ex. 8 3.62 2.400.018 0.007 0.39 0.42 0.035 0.018 0.002 0.005 0.005 Ex. 9 3.59 2.440.023 0.008 0.42 0.38 0.037 0.015 0.002 0.004 0.005 Ex. 10 3.61 2.390.016 0.010 0.40 0.37 0.033 0.013 0.002 0.003 0.005 Ex. 11 3.58 2.370.020 0.008 0.41 0.43 0.032 0.012 0.001 0.002 0.004 Ex. 12 3.63 2.420.021 0.007 0.39 0.40 0.036 0.011 0.001 0.001 0.004 Ex. 13 3.66 2.950.019 0.011 0.75 0.42 0.058 0.038 0.008 0.009 0.007 Ex. 14 3.86 2.980.045 0.019 0.40 0.43 0.021 0.037 0.007 0.008 0.007 Ex. 15 3.31 2.090.048 0.018 0.39 0.37 0.022 0.035 0.008 0.007 0.007 Ex. 16 3.32 2.890.020 0.008 0.76 0.50 0.053 0.036 0.007 0.007 0.008 Com. 1 3.62 2.410.017 0.011 0.39 0.39 0.038 0.053 0.003 0.005 0.009 Com. 2 3.57 2.420.022 0.008 0.38 0.37 0.041 0.008 0.005 0.004 0.003 Com. 3 3.61 2.390.021 0.009 0.42 0.38 0.035 0.004 0.002 0.003 0.005 Com. 4 3.58 2.400.018 0.010 0.40 0.41 0.038 0.034 0.0004 0.004 0.004 Com. 5 3.60 2.430.020 0.007 0.37 0.36 0.039 0.036 0.003 0.0003 0.004 Com. 6 3.59 2.380.019 0.008 0.38 0.38 0.042 0.014 0.001 0.002 0.003 Com. 7 3.62 2.350.019 0.008 0.36 0.39 0.037 0.065 0.002 0.002 0.007 Com. 8 3.61 2.410.021 0.009 0.44 0.39 0.036 0.027 0.024 0.001 0.006 Com. 9 3.60 2.400.022 0.012 0.37 0.42 0.040 0.030 0.001 0.023 0.007 Com. 10 3.58 2.390.018 0.010 0.35 0.41 0.039 0.037 0.002 0.005 0.012 Com. 11 3.57 2.380.020 0.008 0.38 0.40 0.043 0.051 0.001 0.020 0.008 Ref. 3.61 2.41 0.0210.009 0.41 0.39 0.045 0.004 0.003 0.0008 0.003 Note: ⁽¹⁾Remnantconsisting of Fe and inevitable impurity

Test pieces were cut from samples of the spheroidal graphite cast ironsobtained by the above-described casting, and the following evaluationswere performed.

(1) Gas Defect Area Ratio

To find out the occurrence tendency of the gas defect of the spheroidalgraphite cast irons of the Examples and the Comparative examples, flattest pieces were made that had a shape where the gas defect was causedmore readily than in the actual products. For this reason, themeasurement values of the gas defect area ratio were extremely higherthan those of the actual products. FIGS. 1A and 1B are schematic viewsof the flat test pieces for measuring the gas defect, FIG. 1A is a planview, and FIG. 1B is a side view. These flat test pieces 10 were 60 mmin width, 150 mm in length and 10 to 15 mm in thickness. The flat testpieces 10 were each obtained by doing as follows: After the melts sameas the one-inch Y blocks were each poured from the sprue at not lessthan 1400 degrees C. into a sand mold defining a cavity formed of theflat test piece 10, a riser 11 with a diameter of 45 mm and a height of60 mm, the sprue (not shown), a runner 12 a with a width of 35 mm and athickness of 3 mm and an ingate 12 b with a width of 40 mm and athickness of 9 mm, cooling and shake-out were performed, the riser 11was cut to be separated, and shot blast processing was performed.

To observe the gas defects on the surface and inside, by usingtransmission X-ray equipment (manufactured by Toshiba Corporation, thetrade name: EX-260GH-3), X-rays were applied from above the flat testpieces (the direction perpendicular to the paper of FIG. 1A) oncondition that the tube voltage was 192 kV and the application time wasthree minutes, and transmission X-ray pictures were taken.

After only the gas defects on the surface and inside were visuallyextracted from each transmission X-ray picture and traced, imageprocessing was performed by using an image analysis device (manufacturedby Asahi Kasei Corporation, the trade name: IP-1000), and the total area(mm²) of the gas defects was measured. The total area of the gas defectswas divided by the whole projected area of the flat test piece to obtainthe gas defect area ratio (%). It is needless to say that the lower thegas defect area ratio, the more excellent as the spheroidal graphitecast iron. The results of the gas defect area ratio measurement areshown in Table 2. Table 2 also shows the total amounts of Ti, V and Nband the values of the expression (1) in the Examples 1 to 16, theComparative examples 1 to 11 and the Reference example.

As is apparent from Table 2, the test pieces of the Examples 1 to 16where the contents of Ti, V, Nb and N were within the composition rangeof the present invention were lower in gas defect area ratio than thetest pieces of the Comparative examples 2 to 6 where the content of oneor more than one element of Ti, V, Nb and N was too small. It wasconfirmed that even in the spheroidal graphite cast iron obtained byusing a melt containing excessive free N, the gas defect occurrencetendency could be reduced by defining the lower limits of the contentsof Ti, V and Nb as described above. In the spheroidal graphite cast ironof the present invention, the gas defect area ratio is preferably notmore than 11%, is more preferably not more than 10.5%, and is mostpreferably not more than 10%.

(2) Tool Life

Turning was performed on end surfaces of cylindrical test pieces with anoutside diameter of 100 mm, an inside diameter of 62 mm and a length of100 mm by using a cemented insert P10 (JIS B 4053) CDV-coated with TiCNunder the following conditions:

Cutting speed: 180 m/min.

Feed: 0.25 mm/rev.

Depth of cut: 2.0 mm

Cutting fluid: water-soluble cutting fluid

In the milling of each cylindrical test piece, it was determined thatthe end of the life was reached when the depth of the flank wear of thecemented insert became 0.3 mm, and the cutting time (minute) until thatwas reached was the tool life. Needless to say, the longer the tool lifeis, the more excellent machinability is.

Since the absolute value of the tool life is affected by the cuttingcondition, the test piece shape and the like, the “tool life improvementrate” was used as the index of the machinability improvement effect notaffected thereby. The tool life improvement rate is a value (A/B)obtained by dividing the tool life A of each of the spheroidal graphitecast irons of the Examples and the Comparative examples by the tool lifeB of the spheroidal graphite cast iron of the Reference examplerepresentative of the conventional technical level. The tool lifeimprovement rates (times) of the Examples 1 to 16, the Comparativeexamples 1 to 11 and the Reference example are shown in Table 2.

As is apparent from Table 2, the tool life improvement rates of theExamples 1 to 16 within the composition range of the present inventionwere all in a range of 1.0 to 1.3 times. From the results of theExamples 1 to 16, it is apparent that the spheroidal graphite cast ironof the present invention has machinability equal to or greater than theconventional one. On the other hand, the tool life improvement rates ofthe Comparative example 1 and the Comparative examples 7 to 11 where thecontent of one or more than one element of Ti, V, Nb and N was too largewere all less than 1.0 time, and machinability was poor. In thespheroidal graphite cast iron of the present invention, the tool lifeimprovement rate is preferably not less than 1.1 times, is morepreferably not less than 1.2 times, and is most preferably 1.3 times.

(3) Tensile Test

A test piece of 14A of JIS Z 2201 was made from a one-inch Y block, atensile test was performed at room temperature by an Amsler tensiletesting machine (AG-IS250kN manufactured by Shimadzu Corporation)according to JIS Z 2241, and the tensile strength, the 0.2% yieldstrength and the elongation were measured. The results are shown inTable 2.

As shown in Table 2, in all of the Examples 1 to 16 within thecomposition range of the present invention, the tensile strength was notless than 600 MPa, the 0.2% yield strength was not less than 350 MPa andthe elongation was not less than 12%, and it was confirmed that all hadmechanical characteristics equal to or greater than the conventionalones shown in the Reference example. On the contrary, in all of theComparative examples 2, 5, 7 and 11 outside the composition range of thepresent invention, the tensile strength was as low as less than 600 MPa,and in all of the Comparative example 1 and the Comparative examples 7to 11 where the content of one or more than one element of Ti, V, Nb andN was too large, the elongation was as low as less than 12%. Moreover,in the case of the Comparative examples 3, 4 and 6 having a tensilestrength of not less than 600 MPa and an elongation of not less than12%, although they had mechanical characteristics, the gas defect arearate was as high as not less than 12.7% in all of them.

As described above, it was confirmed that the spheroidal graphite castiron of the present invention was a spheroidal graphite cast iron havingmechanical characteristics and machinability equal to or greater thanthe conventional ones and further, having excellent gas effectresistance at the same time.

TABLE 2 Evaluation result Ti + V + Nb Gas Tool total defect life 0.2%amount Value of area Improvement Tensile yield (mass expression ratiorate strength strength Elongation No. ratio (%)) (1)⁽²⁾ (%) (times)(MPa) (MPa) (%) Ex. 1 0.022 1.4 9.5 1.3 608 367 13.7 Ex. 2 0.043 1.7 9.21.2 605 358 12.2 Ex. 3 0.059 1.9 8.0 1.1 603 361 13.7 Ex. 4 0.043 2.09.3 1.2 607 365 13.5 Ex. 5 0.049 1.9 9.1 1.1 609 366 13.5 Ex. 6 0.0351.5 9.5 1.2 621 368 13.2 Ex. 7 0.028 1.2 9.9 1.3 615 367 13.1 Ex. 80.025 1.3 9.8 1.4 621 369 13.2 Ex. 9 0.021 1.1 10.1 1.3 604 364 13.8 Ex.10 0.018 1.0 10.7 1.3 602 360 13.7 Ex. 11 0.015 1.0 10.8 1.4 641 37512.5 Ex. 12 0.013 0.9 10.9 1.4 613 367 13.5 Ex. 13 0.055 2.1 8.3 1.0 644419 13.3 Ex. 14 0.052 2.0 8.6 1.1 601 404 14.7 Ex. 15 0.050 1.9 8.8 1.1626 360 12.0 Ex. 16 0.050 1.7 9.4 1.1 690 435 12.0 Com. 1 0.061 1.9 8.50.1 607 363 11.8 Com. 2 0.017 1.4 15.6 1.0 591 357 13.5 Com. 3 0.009 0.412.7 1.3 609 362 13.6 Com. 4 0.038 2.6 13.5 1.1 623 369 13.1 Com. 50.039 2.8 12.9 1.0 592 357 14.0 Com. 6 0.017 1.5 14.4 1.2 602 359 13.8Com. 7 0.069 2.8 8.9 0.1 590 356 10.9 Com. 8 0.052 2.4 9.3 0.3 612 36911.7 Com. 9 0.054 1.8 9.4 0.4 613 370 11.5 Com. 10 0.044 1.0 11.5 0.6614 365 11.8 Com. 11 0.072 2.3 8.7 0.1 572 362 9.5 Ref 0.008 0.7 10.71.0 611 365 13.6 Note: ⁽²⁾Value of expression (1) is value calculated by(0.29Ti + 0.27V + 0.15Nb)/N

It is to be noted that, as used herein and in the appended claims, thesingular forms “a”, “an”, and “the” include plural referents unless thecontext clearly dictates otherwise.

The embodiments disclosed this time are examples in all respects, andshould be considered to be not restrictive. The scope of the presentinvention is not limited to the above-described meaning but is indicatedby the claims, and it is intended that all modifications within themeaning and scope equivalent to the claims are included.

1-5. (canceled)
 6. A spheroidal graphite cast iron excellent in gasdefect resistance consisting of, in mass ratio: C: 3.3 to 4%; Si: 2 to3%; P: not more than 0.05%; S: not more than 0.02%; Mn: not more than0.8%; Cu: not more than 0.8% (0 is not included); Mg: 0.02 to 0.06%; Ti:0.01 to 0.04%; V: 0.001 to 0.01%; Nb: 0.001 to 0.01%; and N: 0.004 to0.008%, with the remnant substantially consisting of Fe and aninevitable impurity.
 7. The spheroidal graphite cast iron according toclaim 6, wherein the spheroidal graphite cast iron contains, in massratio, 0.015 to 0.045% Ti, V and Nb in total and further, contains Ti,V, Nb and N so as to satisfy the following expression (1):0.8≤(0.29Ti+0.27V+0.15Nb)/N≤2.0  (1) here, the element symbols in theexpression (1) represent the contents [mass ratio (%)] of the elementsin the spheroidal graphite cast iron.
 8. The spheroidal graphite castiron according to claim 6, wherein the spheroidal graphite cast ironcontains, in mass ratio, not less than 0.005% P and not less than 0.005%S.
 9. The spheroidal graphite cast iron according to claim 6, whereinthe spheroidal graphite cast iron contains, in mass ratio, not less than0.2% Mn and not less than 0.1% Cu.
 10. The spheroidal graphite cast ironaccording to claim 6, wherein the spheroidal graphite cast iron is notless than 600 MPa in tensile strength and not less than 12% inelongation.
 11. The spheroidal graphite cast iron according to claim 6,wherein a gas defect area ratio of the spheroidal graphite cast iron isnot more than 11%.
 12. The spheroidal graphite cast iron according toclaim 6, wherein a tool life improvement rate of the spheroidal graphitecast iron is not less than 1.0 times.